Method for preparation of ammonia gas and co2 for a urea synthesis process

ABSTRACT

The invention relates to a process for preparing ammonia gas and CO 2  for urea synthesis. In the process of the invention, a process gas containing nitrogen, hydrogen and carbon dioxide as main components is produced from a metallurgical gas. The metallurgical gas consists of blast furnace gas, or contains blast furnace gas at least as a mixing component. The process gas is fractionated to give a gas stream containing the CO 2  component and a gas mixture consisting primarily of N 2  and H 2 . An ammonia gas suitable for the urea synthesis is produced from the gas mixture by means of ammonia synthesis. CO 2  is branched off from the CO 2 -containing gas stream in a purity and amount suitable for the urea synthesis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No. 15/103,875, filed Jun. 12, 2016, which is the national phase of, and claims priority to, International Patent Application No. PCT/EP14/03315, filed Dec. 11, 2014, which designated the U.S. and which claims priority to German Patent Application Number DE 10 2013 113 980.9, filed Dec. 12, 2013. Each of these applications is incorporated by reference herein in its entirety.

BACKGROUND

1. Field of the Invention

The invention relates to a process for preparing ammonia and CO₂ for urea synthesis.

2. Description of Related Art

Industrially, urea is obtained from NH₃ and CO₂ via the intermediate ammonium carbamate. The ammonium carbamate is formed rapidly and completely when dissociation is avoided by means of a sufficiently high reaction pressure. The exothermically formed ammonium carbamate is converted endothermically into urea in subsequent decomposition stages at low pressure, with excess gases being able to be recirculated back to the reactor. The reaction to form ammonium carbamate is carried out using an excess of NH₃, with a molar ratio of NH₃/CO₂ of about 4 frequently being selected in practice.

Raw materials for the urea synthesis are CO₂ and NH₃. Since carbon dioxide is obtained as a secondary component in the synthesis of ammonia, a urea plant is frequently operated in conjunction with an ammonia plant. Plants which ultimately produce urea from natural gas synthesize ammonia from natural gas and air, and then synthesize urea from this ammonia and carbon dioxide.

SUMMARY

In view of the background above, it is an object of the invention to provide an efficient process for producing the gaseous starting materials for urea synthesis. To operate the process, a raw gas which is obtained as a waste product in an industrial process should be utilized. The raw gas and the process steps should be selected so that the gas components of the raw gas are substantially completely converted into ammonia and CO₂ in the proportions necessary for the urea synthesis.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic block diagram illustrating a process for preparing gaseous starting materials for synthesis of urea.

DETAILED DESCRIPTION

According to the invention, a metallurgical gas which contains blast furnace gas at least as a mixing component, or which consists of blast furnace gas, is used for preparing the gaseous starting materials for urea synthesis. Blast furnace gas is obtained in the production of pig iron in a blast furnace. In the blast furnace, pig iron is obtained from iron ores, additives, and coke and other reducing agents such as coal, oil or gas. As products of the reduction reactions, CO₂, hydrogen and water vapor are inevitably formed. A blast furnace gas taken from the blast furnace process has, in addition to the abovementioned constituents, a high content of nitrogen. The composition of the blast furnace gas is dependent on the feedstocks and the mode of operation, and is subject to fluctuations. However, blast furnace gas usually contains from 35 to 60% by volume of N₂, from 20 to 30% by volume of CO, from 20 to 30% by volume of CO₂ and from 2 to 15% by volume of H₂.

Furthermore, a metallurgical gas which consists of a mixed gas composed of blast furnace gas and converter gas, or of a mixed gas composed of blast furnace gas, converter gas and coke oven gas can be used for the process of the invention. Converter gas, which is created from pig iron during the steel production process, has a high content of CO, and also contains nitrogen, hydrogen and CO₂. A typical converter gas composition has from 50 to 70% by volume of CO, from 10 to 20% by volume of N₂, about 15% by volume of CO₂ and about 2% by volume of H₂. Coke oven gas is obtained in the coking of coal and has a high hydrogen content and appreciable amounts of CH₄. Coke oven gas typically contains from 55 to 70% by volume of H₂, from 20 to 30% by volume of CH₄, from 5 to 10% by volume of N₂ and from 5 to 10% by volume of CO. The coke oven gas additionally comprises CO₂, NH₃ and H₂S.

In the process of the invention, a process gas containing nitrogen, hydrogen and carbon dioxide as main components is produced from the metallurgical gas and this process gas is subsequently fractionated to give a gas stream containing the CO₂ component and a gas mixture consisting primarily of N₂ and H₂. An ammonia gas suitable for the urea synthesis is produced from the gas mixture by means of ammonia synthesis. CO₂ is branched off from the CO₂-containing gas stream in a purity and amount suitable for the urea synthesis. The conditioning of the metallurgical gas and the separation steps described can be matched to one another in such a way that ammonia and CO₂ are formed in the proportions necessary for the urea synthesis and the metallurgical gas can be utilized almost completely for preparing the gaseous starting materials required for the urea synthesis.

The use of the metallurgical gas for producing process gas is advantageously preceded by a gas purification process. The gas purification process serves to separate undesirable constituents, in particular tar, sulfur and sulfur compounds, aromatic hydrocarbons (BTX) and high-boiling hydrocarbons.

The CO component of the metallurgical gas can be converted into CO₂ and H₂ by means of a water gas shift reaction, forming a process gas which contains nitrogen, hydrogen and carbon dioxide as main components.

The process gas is subsequently fractionated, preferably by means of pressure swing adsorption (PSA), to give a gas mixture consisting primarily of nitrogen and hydrogen and an offgas, also referred to as PSA offgas, containing the CO₂ component. Pressure swing adsorption (PSA), which is known in the prior art, is used for the isolation and purification of hydrogen. In the context of the process of the invention, the pressure swing adsorption is operated in combination with a preceding gas conditioning process in such a way that a desired concentration ratio of H₂ and N₂ is established. One aspect of the process of the invention is therefore the coupling of a gas conditioning process, in particular a water gas shift reaction, with a pressure swing adsorption in order to produce a synthesis gas suitable for the ammonia synthesis from metallurgical gas which contains blast furnace gas at least as a mixing component, or which consists of blast furnace gas. Furthermore, secondary components which are unfavorable for the ammonia synthesis, e.g. argon, methane or carbon monoxide, can be removed or have their concentrations reduced by means of the pressure swing adsorption.

The pressure swing adsorption produces an energy-rich offgas (PSA offgas) which contains the CO₂ component of the process gas and any residual proportions of CO. CO₂ for the urea synthesis is obtained from the PSA offgas. In a preferred embodiment of the process of the invention, the CO₂ component is separated from the pressure swing adsorption offgas (PSA offgas) and is subsequently separated into a gas containing a high concentration of CO₂ for the urea synthesis and a tailgas having a lower concentration of CO₂.

The invention also provides a process for preparing urea, in which ammonium carbamate is produced from ammonia gas and CO₂ using an excess of ammonia and this ammonium carbamate is dissociated into water and urea. According to the invention, the ammonia gas required for the synthesis of urea and the CO₂ which is likewise required for the synthesis of urea are each produced from a metallurgical gas which contains blast furnace gas at least as a mixing component, or which consists of blast furnace gas. It is essential for the process of the invention, according to an embodiment, that the gaseous starting materials for the urea synthesis are obtained entirely from the metallurgical gas. The gaseous starting materials for the urea synthesis are obtainable by the process described further above.

The invention will be illustrated below with reference to FIG. 1., which depicts merely one working example. The single figure schematically shows, in the form of a greatly simplified block diagram, a process for preparing gaseous starting materials for a urea synthesis.

A process gas 2 containing nitrogen (N₂), hydrogen (H₂) and carbon dioxide (CO₂) as main components is produced from a metallurgical gas 1 which contains blast furnace gas at least as a mixing component and in the working example consists of blast furnace gas by means of the process depicted in the figure.

The blast furnace gas 1 has, for example, a typical composition of 50% by volume of N₂, 24% by volume of CO₂, 21% by volume of CO and about 4% by volume of H₂. After a gas purification process 3 in which undesirable constituents, for example tar, sulfur and sulfur compounds, aromatic hydrocarbons (BTX) and high-boiling hydrocarbons are separated, the metallurgical gas 1 consisting of blast furnace gas is converted by means of a gas conditioning process 4 into the process gas 2 which consists mainly of N₂, H₂ and CO₂. The gas conditioning process 4 includes, in particular, a CO conversion in which the CO component of the metallurgical gas 1 is converted into CO₂ and H₂ by means of a water gas shift reaction:

CO+H₂O→CO₂+H₂.

After the conversion or the water gas shift reaction, the process gas has a composition of about 37% by volume of CO₂, 21% by volume of H₂ and 42% by volume of N₂.

The process gas 2 is fractionated by means of pressure swing adsorption (PSA) 16 to give a gas mixture 5 consisting primarily of N₂ and H₂ and an offgas 6 containing the CO₂ component. An ammonia gas 8 suitable for the synthesis of urea is produced from the N₂- and H₂-containing gas mixture by means of an ammonia synthesis 7. In the ammonia synthesis 7, the gas mixture composed of hydrogen and nitrogen can, for example, be reacted over an iron oxide mixed catalyst at pressures in the range from 150 to 200 bar and at a reaction temperature of from 350 to 550° C.

CO₂ for the urea synthesis 9 is obtained from the offgas 6 from the pressure swing adsorption. According to the process scheme depicted in the figure, the CO₂ component 11 is separated from the offgas 6 from the pressure swing adsorption in a first separation stage 10. Subsequently, a separation into a gas 13 containing a high concentration of carbon dioxide and a tailgas 14 having a low concentration of CO₂ is carried out in a second separation stage 12. The gas 13 is, in particular, carbon dioxide in a purity necessary for the urea synthesis.

CO₂ and NH₃ are fed to the urea plant in the proportions required for the urea synthesis 9. In the urea plant, ammonium carbamate is produced using an excess of ammonia and this ammonium carbonate is converted into urea 15 in subsequent decomposition stages at low pressure.

The process illustrated in FIG. 1 can also be operated using a gas mixture of blast furnace gas and converter gas or using a gas mixture of blast furnace gas, converter gas and coke oven gas as metallurgical gas 1. 

1. A process for preparing urea, where ammonium carbamate is produced from ammonia gas and CO₂ using an excess of ammonia and the ammonium carbamate is converted into urea in subsequent decomposition stages at low pressure, wherein the ammonia gas required for the synthesis of urea and the CO₂ which is likewise required for the synthesis of urea are produced from a metallurgical gas which contains blast furnace gas at least as mixing component or consists of blast furnace gas. 